IDENTITY OF LIGHT AND ELECTRICITY.
Professor Clerk Maxwell, in 1864, laid down the proposition that light and electricity were one and the same thing ; that all manifestations of light, radiant heat, and similar phenomena were electro magnetic vibrations of the luminiferous ether travelling in the form of undulatory waves (see Fig. 556). This conclusion has now been endorsed by the scientific world, especially since its experimental demon stration in 1SS8 by Professor Hertz. Electric waves have been proved to be capable of reflection, refraction, diffrac majority of compounds, owing to its brownish black colour. Various resins and gums show sensibility. Pliny, in the first century A.D., referred to the bleach ing of yellow wax by sunlight. Hog's -fat and palm oil also are slightly sensitive. In the organic kingdom there are very few bodies which are not affected by prolonged exposure to light. The action of light in promoting the speedy crystallisation of chemical solutions early attracted atten tion. The tanning effect of sunlight on
the skin, and the marked consequences of its absence in the vegetable and animal tion, and polarisation in precisely the same way as light waves ; although, being of much greater length than the latter, they cannot be seen by the eye. As shown by the Röntgen rays, electricity is capable of affecting the photographic plate in a similar manner to light ; conversely, light has the power in various ways of in fluencing and modifying electrical phe nomena. A remarkable illustration of a mechanical effect produced by the action of light is afforded by Crookes' radio meter (Fig. 558), which consists of a glass bulb from which the air is exhausted, and are balanced with extreme deli acy four very light vanes, blackened on one side only. On exposure, the blackened sides of the vanes move away from the light, thus setting up what be comes in sunlight a rapid revolution.